A number of non-destructive inspection techniques are in widespread use. These non-destructive inspection techniques allow a part to be inspected for anomalies without sectioning or etching the part, and without otherwise altering the structure of the part, which procedures in themselves add further anomalies to the part. Examples of non-destructive inspection techniques include ultrasonic testing, surface acoustic techniques, and eddy current inspection techniques. These techniques may be used to inspect new-make parts and parts that have previously been in service.
In the eddy current technique, a high-frequency alternating magnetic field applied at the surface of the part produces a responsive pattern of high-frequency electrical eddy currents within the part. The electrical eddy currents produce their own induced magnetic fields, which can be detected externally. The electrical eddy currents and their associated induced magnetic fields are normally regular in pattern, but the regularity is disrupted by the presence of anomalies in the part. Examples of anomalies include cracks, incipient cracks, inclusions at or near the surface, particles at or near the surface, and the like. By externally detecting the pattern of induced magnetic fields and their irregularities, the presence, size, and other features of the anomalies are deduced.
Eddy current inspection techniques have the important advantage that they allow the near-surface region of the part to be inspected nondestructively. Inspection of the near-surface region is important, because some mechanisms that produce premature failure of the part initiate at the surfaces of the part. In particular, anomalies such as cracks often initiate from surface edges or other non-planar regions of the part, where there are structural irregularities and/or stress concentrations. After surface-edge crack initiation, the cracks propagate into and through the remainder of the part, possibly resulting in a premature failure. Examples of such non-planar surface-edge crack-initiation sites include machined or cast edges between a front and a side of the part, intentionally produced holes such as fastener holes or large bores, intentionally produced cutouts such as the dovetail slots on the periphery of a turbine disk, and openings such as cooling holes.
The eddy current technique is used to detect the presence of the anomalies in as-manufactured (new-make) parts, and also in those parts after they have been in service, by periodic inspections. If no relevant indications of anomalies of a critical size are detected, the part may be placed into, or continued in, service. If anomalies of a critical size or larger are detected, the part is not continued in service, and is either repaired or scrapped.
One of the limitations on the use of the eddy current technique is the ability to discern an anomaly in the midst of background noise, a characteristic often expressed as the signal-to-noise ratio. For an anomaly to be reliably detected by the eddy current technique, the signal-to-noise ratio of the anomaly must be sufficiently high that the anomaly is not confused with the background noise. Too low a signal-to-noise ratio of a particular kind of anomaly means that the anomaly cannot be reliably detected.
Non-planar surfaces of the part are significant sources of noise in the analysis of eddy current output signals. Because these non-planar surfaces are the common locations where cracks or other anomalies may often be found, the noise associated with the non-planar surfaces of the part may mask the embryonic anomalies. The value of the eddy current technique is thereby lessened.
There is a great need for a technique by which eddy current detection of non-planar disturbances in the non-planar regions of parts may be accomplished with an increased signal-to-noise ratio as compared with current approaches, achieving more reliable detection of the anomalies. The present invention fulfills this need, and further provides related advantages.